The present invention is directed to a wavefront sensor and more particularly to a wavefront sensor for ophthalmic applications.
It is known in astronomy to detect wavefront aberrations caused by the atmosphere through the use of a Hartmann-Shack detector. Such detection is disclosed, e.g., in D. M. Alloin and J. M. Mariotti, eds., Adaptive Optics for Astronomy, Dordrecht: Kluwer Academic Publishers, 1994. More recently, such a technique has been used to detect wavefront aberrations in the human eye for such purposes as intraocular surgery and contact lens fabrication. Such detection is disclosed, e.g., in Liang et al, "Objective measurement of wave aberrations of the human eye with the user of a Hartmann-Shack wave-front sensor," Journal of the Optical Society of America, Vol. 11, No. 7, July, 1994, pp. 1-9, the disclosure of which is hereby incorporated by reference in its entirety into the present disclosure.
That technique will be summarized with reference to FIG. 1. A beam of light from a laser diode or other light source is directed toward the pupil and is incident on the retina. Because the retina is highly absorbing, a beam on the order of four orders of magnitude dimmer than the original beam is reflected by the retina and emerges from the pupil. Typically, the incoming and emergent light follow a common optical path; the incoming light is brought into the common optical path with a beamsplitter.
The emergent beam is applied to a Hartmann-Shack sensor to detect the aberrations. Such a detector includes an array of lenslets that break up the light into an array of spots and focus the spots onto a charge-coupled detector or other two-dimensional light detector. Each spot is located to determine its displacement A from the position which it would occupy in the absence of wavefront aberrations, and the displacements of the spots allow reconstruction of the wavefront and thus detection of the aberrations through known mathematical techniques.
Improvements to the technique of Liang et al are taught in J. Liang and D. R. Williams, "Aberrations and retinal image quality of the normal human eye," Journal of the Optical Society of America, Vol. 4, No. 11, November, 1997, pp. 2873-2883 and in U.S. Pat. No. 5,777,719 to Williams et al. The disclosures of that article and of that patent are hereby incorporated by reference in its entirety into the present disclosure. Williams et al teaches techniques for detecting aberrations and for using the aberrations thus detected for eye surgery and the fabrication of intraocular and contact lenses. Moreover, the techniques of those references, unlike that of the Liang et al 1994 article, lend themselves to automation. German published patent application No. DE 42 22 395 A1 teaches a further variation using polarizing optics to control back-scatter from the lenses in the detector setup.
Analysis of the eye presents unique problems and issues not necessarily faced in astronomy. For example, while wavefront sensor systems in astronomy exhibit uniform intensity across their entrance pupil, this is not the case with systems for the eye. The eye, unlike a telescope, is subject to the Stiles-Crawford effect. That effect is a directional sensitivity of the retina, one manifestation of which is an intensity variation across the pupil of the eye when light is reflected from the retina. It exists because illuminated cones radiate more light back toward the center of the pupil than toward the pupil margin. Also, unlike astronomy, stray light from other sources, such as from corneal reflection, can be introduced into the wavefront sensor from the eye, and such stray light can interfere with the measurement of spot placement.
Other problems unique to the eye have been encountered. For example, a subset of the spots that should be formed in the Hartmann-Shack detector cannot be seen, either because the aberrations are unusually large (e.g., a huge aberration caused by a scar or the like can displace or deform the spot so much that the spot's origin cannot be determined or the spot leaves the field of view of the detector altogether), or they are occluded by opacities in the eye's optics or by the pupil. In current wavefront sensors, the loss of any spots frustrates the computation of the wave aberration.
Another problem is that of variable pupil size, as opposed to the fixed pupil of a telescope.
Moreover, there is the issue of real-time operation. Real-time wavefront sensors have been demonstrated in astronomy, but where operation is required at rates typically greater than 300 Hz with closed loop bandwidths greater than 30 Hz. The atmosphere is much too turbulent for real-time compensation at slower rates. On the other hand, present adaptive optics techniques for the eye operate at a very slow rate, less than 0.25 Hz, and do not automatically compute the wave aberration, even with single exposures. Real-time operation is not possible because of the factors described above. Also, these techniques employ relatively long focal length lenslet arrays. Such instruments have high sensitivity to small changes in the slope of the wave aberration at the expense of dynamic range and robustness. Individual spots in the wavefront sensor image often overlap, particularly near the edge of the pupil, making it difficult for automatic centroid spot computation. Such problems can develop especially for a commercial instrument in which operator intervention should be minimized. As a result, these systems cannot measure the wave aberration in a large fraction of human eyes. An optimized wavefront sensor for the eye should therefore properly balance sensitivity and dynamic range, operate in real-time, and be capable of use with a significant fraction of eyes.
The measurement sensitivity of a wavefront sensor for the eye is determined primarily by the focal length of the lenslet array. The smallest measurable wavefront slope is proportional to the focal length. Relatively long focal length (e.g., 97 mm) lenslet arrays used in high sensitivity wavefront sensors for measuring eye wave aberrations typically show a small mean standard deviation of repeated measurements across the pupil of an artificial eye, for example, .lambda./487 (at 632.8 nm, the helium-neon laser wavelength) for a 3.4 mm pupil. Moreover, the eye can exhibit a severe wave aberration at the edge of the pupil due to smeared or scarred areas of the eye's tissue. Thus, such wavefront sensors exhibit more sensitivity than necessary and require an algorithm to hunt/locate the migrating spots.
Another challenge to real-time wavefront sensing in the eye is the spatial homogeneity of the spot of light imaged on the retina. Inhomogeneity, caused, for example, by laser speckle or reflectance variations in the underlying retina, disrupts the accuracy of the spot localizatoin. This problem is exacerbated with the short exposures required for real-time operation.
As a result of the above-noted problems with wavefront sensing in the eye, a robust and real-time sensing technique for the eye is not known in the art.